This invention relates to photonic devices in general, and more particularly to tunable lasers.
In pending prior U.S. patent application Ser. No. 09/105,399, filed Jun. 26, 1998 by Parviz Tayebati et al. for MICROELECTROMECHANICALLY TUNABLE, CONFOCAL, VERTICAL CAVITY SURFACE EMITTING LASER AND FABRY-PEROT FILTER, and in pending prior U.S. patent application Ser. No. 09/543,318, filed Apr. 5, 2000 by Peidong Wang et al. for SINGLE MODE OPERATION OF MICROMECHANICALLY TUNABLE, HALF-SYMMETRIC, VERTICAL CAVITY SURFACE EMITTING LASERS, which patent applications are hereby incorporated herein by reference, there are disclosed tunable Fabry-Perot filters and tunable vertical cavity surface emitting lasers (VCSEL""s).
More particularly, and looking now at FIG. 1, there is shown a tunable Fabry-Perot filter 5 formed in accordance with the aforementioned U.S. patent applications Ser. Nos. 09/105,399 and 09/543,318. Filter 5 generally comprises a substrate 10, a bottom mirror 20 mounted to the top of substrate 10, a bottom electrode 15 mounted to the top of bottom mirror 20, a thin support 25 atop bottom electrode 15, a top electrode 30 fixed to the underside of thin support 25, a reinforcer 35 fixed to the outside perimeter of thin support 25, and a confocal top mirror 40 set atop thin support 25, with an air cavity 45 being formed between bottom mirror 20 and top mirror 40.
As a result of this construction, a Fabry-Perot filter is effectively created between top mirror 40 and bottom mirror 20. Furthermore, by applying an appropriate voltage across top electrode 30 and bottom electrode 15, the position of top mirror 40 can be changed relative to bottom mirror 20, whereby to change the length of the Fabry-Perot cavity, and hence tune Fabry-Perot filter 5.
Correspondingly, and looking next at FIG. 2, a tunable vertical cavity surface emitting laser (VCSEL) 50 can be constructed by positioning a gain medium 55 between bottom mirror 20 and bottom electrode 15. As a result, when gain medium 55 is appropriately stimulated, e.g., by optical pumping or by electrical pumping, lasing can be established between top mirror 40 and bottom mirror 20. Furthermore, by applying an appropriate voltage across top electrode 30 and bottom electrode 15, the position of top mirror 40 can be changed relative to bottom mirror 20, whereby to change the length of the laser""s resonant cavity, and hence tune laser 50.
The present invention is directed to tunable lasers of the type disclosed in the aforementioned U.S. patent applications Ser. Nos. 09/105,399 and 09/543,318.
Tunable lasers of the type disclosed in the aforementioned U.S. patent application Ser. Nos. 09/105,399 and 09/543,318 are highly advantageous since they can be quickly and easily tuned by simply changing the voltage applied across the top electrode and the bottom electrode.
However, it has been found that tunable lasers of the type disclosed in the aforementioned U.S. patent application Ser. Nos. 09/105,399 and 09/543,318 can suffer from vibrational problems.
The aforementioned vibrational problems may be due to a variety of factors, such as thermal noise; or noise in the tuning voltage of the laser; or, in the case of an electrically pumped laser, shot noise in the injection current; etc.
Regardless of the cause, the effect of these vibrational problems is to cause the laser to move out of tune. In other words, these vibrational effects cause the output frequency of the laser to change even though the tuning voltage of the laser is held constant. While the extent of this vibration-related frequency shift may be relatively modest (e.g., a 300 MHz shift in the lasing frequency from a 100 MHz vibration frequency), this frequency shift may nonetheless create significant problems in certain types of systems, e.g., WDM communication systems.
See, for example, FIG. 3, which schematically illustrates how the aforementioned vibrational problems may cause a relatively periodic modulation of the lasing frequency; and FIG. 4, which schematically illustrates how the aforementioned vibrational problems may cause a relatively irregular modulation of the lasing frequency.
As a result, an object of the present invention is to provide a method and apparatus for stabilizing the wavelength of tunable lasers affected by the aforementioned vibrational problems.
The present invention provides a fast and easy way to compensate for the aforementioned vibrational problems in tunable lasers, by correspondingly adjusting the electrooptical performance of the laser""s gain medium, whereby to eliminate the frequency shift due to vibrational factors.
The electrooptical performance of the laser""s gain medium is adjusted, in the case of an electrically pumped laser, by changing the injection current used to pump the laser; and the electrical performance of the laser""s gain medium is adjusted, in the case of an optically pumped laser, by changing the intensity of the pump laser used to energize the laser.
The system is implemented with a feedback mechanism. A wavelength measuring module detects the difference between the instantaneous wavelength of the laser and the desired wavelength of the laser, and generates a voltage signal which is representative of this difference. This voltage signal is then used to appropriately modify the electrooptical performance of the laser""s gain medium, either by appropriately adjusting the injection current applied to the gain medium (in the case of an electrically pumped laser), or by appropriately adjusting the intensity of the pump laser applied to the gain medium (in the case of an optically pumped laser).